Developing in the dry-type electrophotographic method is the most important element which has influence directly on the image quality, and various methods such as the cascade method and the magnet brush method have conventionally been used. The reason is that, in these developing methods, toner is easly charged and a stable image can be obtained.
However, in the cascade method, it is impossible to copy the solid part of an image satisfactorily due to fringe effects at developing. In the magnet brush method, such defects are few. The magnet brush method as commonly used, however, has the defect that the life of the carrier particles used with the toner particles is comparatively short. Therefore, it is necessary to replace the carrier particles every time after copying ten thousands of pictures.
For removing such defects in these methods, a system where the shape and the material of the carrier particles were improved has appeared recently, but it is not sufficient. Under the circumstances, a copying machine using the so-called magnetic one component developing system has been introduced. This system is classified into two systems, one using a conductive toner and the other using an insulating toner. For the requirement of improving moisture proofness, it is the technical target to select the latter. The expression of the target means that some problems still remain in this technology. The source of the problems is generated because toner can not be charged stably. For this reason, defects of an image such as uniformity, background, and density are produced causing an obstacle to a copying machine high in completeness.
Also, as another advantage of the one component system, it is possible to develop in noncontacting state with the electrostatic latent image surface. This advantage is an important element in color copy technology where overlap developing is required. However, so far as magnetic toner is used, it is difficult to make toner colored. The reason is that most magnetic powders that might be used for toner are black or brown, and the color of the toner becomes remarkably muddy when mixed.
For satisfying the above requirement, the necessity of a developing system where an image is produced on the electrostatic latent image surface by using non-magnetic toner in noncontacting state has been increased. However, this involved many unsatisfactory elements in this non-magnetic one component developing system when non-magnetic toner is practically used for copying machines. One of the well-known techniques is a method where an image is developed by forming a thin layer of toner on the surface of a conductive roller and making it face a developing surface having an electrostatic latent image at a space of 600 um or less, preferably 250 um or so. This method, however, has the following two disadvantages. First, there is no reliable means for forming a thin layer (50 .mu.m or less) of toner. At present, a method of applying toner by pressing a rubber blade against a roller is generally known, but there remains such problems as density of the toner layer to be formed and wear of the blade. Second, the formed thin layer of toner must be charged uniformly. In this means, the charging rate of toner is low, a satisfactory charging and stability can not be obtained, and the image quality also is unstable. A trial was made for improving the charging quality and the film forming quality of the toner, but improvement of the toner is put under a large burden. This means, therefore, is at the experimental stage in laboratories. The same problem also for developing toner materials to improve the charging nature on the roller surface still remains.
The conventional developing device (see, e.g., U.S. Pat. No. 4,383,497) comprises a magnetic roller to form a magnetic brush with a two component developer consisting of magnetic carrier particles which electrostatically attract toner particles and a developing roller which passes by the magnetic roller and contacts the magnetic brush such that the toner particles are separated from the magnetic carrier particles and then the toner particles are received on the developing roller, and which passes a developing station where the development is affected. However, in this conventional device, a satisfactory and stable image quality was obtained, but an unfavorable defect was generated that toner is scattered in the copying machine when toner is separated from the two component developer.